1. Field of the Invention
The present invention relates to photographic cameras, and more particularly, to apparatus for moving a lens element in such cameras.
2. Discussion Related to the Problem
Various schemes have been proposed for automatically determining the required focus adjustment in a photographic camera. A feature common to these schemes is a means for moving a lens element in the camera either during or after focus determination. Other cameras include "zoom" lenses, wherein the magnification of the camera lens is changed by moving an element in the camera lens. Copending U.S. patent application Ser. No. 193,771 entitled "Camera With Electronic Flash and Piezoelectric Lens Motor" by L. F. Frank and J. K. Lee filed Oct. 3, 1980 discloses the use of a piezoelectric motor to move a lens element in a photographic camera having a built-in electronic flash. In cameras having built-in electronic flash, the moderately high voltage power supply for the electronic flash can be employed to power the piezoelectric motor, thereby making the piezoelectric focus motor highly attractive in such cameras. The above-referenced application discloses the use of a piezoelectric motor made of lead zirconate titanate (PZT). These motors rely on the inherent stiffness of PZT to hold a lens element in a camera without sagging appreciably under the influence of gravity.
There has been considerable recent interest in another kind of piezoelectric material called polyvinylidene fluorode (PVF.sub.2). PVF.sub.2 is a plastic that can be formed and worked in much the same way as other plastics. It is known, for instance, that a PVF.sub.2 bender deflects much further under a given electric field than does a PZT bender, thereby eliminating the need for further mechanical amplification of the motion in certain applications of the bender. Since PVF.sub.2 can be worked in ways known in plastic manufacturing arts, it would appear to be an ideal piezoelectric material for use in a piezoelectric focus motor. The problem with PVF.sub.2 is that it is not nearly as stiff as PZT and a reasonably rigid cantilever beam of PVF.sub.2 will not support the weight of a lens element without sagging to an undesirable extent under the influence of gravity.